Improving fuel efficiency of automobiles has been an important task in recent years from the viewpoint of global environment protection. Due to this, there has been vigorous trend toward making vehicle body parts thin by increasing the strength of vehicle body materials to reduce the weight of vehicles.
In general, the proportion of a hard phase such as martensite and bainite with respect to the entire microstructure of a steel sheet must be increased to increase the strength of the steel sheet. However, enhancing strength of a steel sheet by increasing the proportion of a hard phase thereof tends to deteriorate formability of the steel sheet. Therefore, there has been a demand for developing a steel sheet having both high strength and good formability in a compatible manner. There have been developed up to now various types of multi-phase steel sheets such as ferrite-martensite dual phase steel (DP steel), TRIP steel utilizing transformation-induced plasticity of retained austenite, and the like.
In a case where the proportion of a hard phase is increased in a multi-phase steel sheet, formability of the steel sheet is strongly influenced by formability of the hard phase not because of the deformability of polygonal ferrite, but deformability of the hard phase itself directly affects formability of the steel sheet in such a case. In contrast, deformability of polygonal ferrite dominates formability of a steel sheet, thereby ensuring good formability, e.g., good ductility, in spite of poor formability of a hard phase in a case where the steel sheet contains soft polygonal ferrite at a relatively high content and the hard phase at relatively low content.
In view of this, there have conventionally been attempts to: subject a cold rolled steel sheet to a thermal treatment to adjust the content of polygonal ferrite generated by an annealing and cooling process thereafter; allow martensite to be formed by water-quenching the steel sheet thus treated; and tempered martensite by heating the steel sheet to relatively high temperature and retaining the steel sheet in that state to allow carbides to be formed in martensite as a hard phase, thereby improving formability of martensite. In such a case of employing facilities for continuous annealing and water-quenching as described above, which inevitably involves water-quenching, however, temperature of a steel sheet after quenching naturally drops to a temperature around the water temperature and most of non-transformed austenite experiences martensitic transformation, whereby it is difficult to utilize low-temperature transformed microstructure such as retained austenite and the like. In other words, improvement of formability of a hard microstructure totally depends on an effect caused by martensite tempering and thus improvement of formability of a steel sheet is significantly limited in the case of employing facilities for continuous annealing and water-quenching.
Alternatively, there has been proposed as a steel sheet having a hard phase other than martensite. The steel sheet includes polygonal ferrite as a main phase and bainite and pearlite as hard phases with carbides formed in bainite and pearlite as the hard phases. This steel sheet aims to improve formability thereof not only by use of polygonal ferrite as the main phase, but also by formation of carbides in the hard phases to improve formability in particular stretch-flangeability of the hard phases themselves. However, it is difficult to make good formability with high tensile strength (TS) exceeding 1180 MPa with this steel sheet as long as it essentially includes polygonal ferrite as the main phase thereof.
Regarding a multi-phase steel sheet containing retained austenite, JP-A 04-235253 proposes a high tensile strength steel sheet having excellent bendability and impact properties, manufactured by specifying alloy components and obtaining steel microstructure constituted of fine and uniform bainite having retained austenite.
JP-A 2004-076114 proposes a multi-phase steel sheet having excellent bake hardenability, manufactured by specifying types and contents of alloy components, obtaining steel microstructure constituted of bainite having retained austenite and controlling the content of the retained austenite in bainite.
Further, JP-A 11-256273 proposes a multi-phase steel sheet having excellent impact resistance, manufactured by specifying types and contents of alloy components, obtaining steel microstructure including at least 90% (by area ratio) bainite having 1% to 15% retained austenite in bainite and setting hardness (HV) of bainite in a specific range.
The aforementioned steel sheet, however, has problems described below.
The component composition described in JP-A 04-235253 cannot ensure a content of stable retained austenite sufficient to express a TRIP effect in a high strain region of a resulting steel sheet when the steel sheet is imparted with strain, whereby the steel sheet exhibits poor ductility prior to reaching plastic instability and poor stretchability, although bendability thereof is relatively good.
The steel sheet of JP-A 2004-076114, although it has good bake hardenability, has difficulties not only in achieving tensile strength (TS) exceeding 1180 MPa, but also in ensuring satisfactory formability when strength thereof is increased due to its microstructure primarily containing bainite and, optionally, ferrite with martensite reduced as best as possible.
The steel sheet of JP-A 11-256273 primarily aims at improving impact resistance and microstructure thereof includes as main phase bainite having hardness (HV) of 250or less by a content exceeding 90%, whereby it is very difficult to achieve tensile strength (TS) exceeding 1180 MPa with this steel sheet.
It is reasonably assumed that a steel sheet for use as a material of vehicle parts requiring high strength such as a door impact beam, a bumper reinforcement that suppresses deformation during car collisions, among automobile parts to be formed by press-forming, generally requires tensile strength (TS) of at least 1180 MPa and, in the future, possibly at least 1470 MPa. Automobile structural members having relatively complicated shapes such as a center pillar inner generally requires tensile strength of at least 980 MPa and, in the future, possibly at least 1180 MPa.